In fine patterning of semiconductor devices, fine patterning relying on only optical lithography technology whose limit is being reached is moving to a new stage thanks to double patterning technology based on conventional optical lithography technology. Various kinds of double patterning technology have been developed/published and the currently mainstream technology includes a sidewall process.
The sidewall process is a process in which a line & space (L/S) pattern with a double pitch is formed by optical lithography technology and the pattern is slimmed and used as a core material to form a sidewall film on the sidewall thereof. Then, the core material is removed and a desired pitch pattern is formed by etching a desired processed film using the remaining sidewall film or a foundation film processed based on the sidewall film as a hard mask. Though the above sidewall process has a long process flow and is complicated when compared with a normal single exposure process, a pattern by lithography only needs to be formed with double the desired pitch and added processes for slimming and sidewall formation are diverted from conventional technology and therefore, finer patterning can advantageously be developed easily while curbing investment.
A major feature of pattern formation by the sidewall process is that the line width is controlled by controlling the thickness of a sidewall film. In a process of forming a pattern of only a simple line & pattern, the pattern can be formed by controlling the width of a core material and the thickness of a sidewall film. However, wiring layers in upper and lower layers are connected to a wiring pattern in an actual semiconductor device via a hole pattern to configure a semiconductor circuit and thus, it is necessary to arrange a wide wiring pattern in places in a pattern layout. Because, as described above, the wire width that can be formed by the sidewall process is determined by the thickness of a sidewall film, such a wide pattern is normally added separately after undergoing the sidewall process.
However, problems that are not considered when various pattern variations are formed by conventional optical lithography are caused by adding the wide pattern. For example, a case when a wide pattern is exposed and developed after being aligned with a sidewall pattern using optical lithography and then, a lower-layer film is etched by using a film pattern configured by the wide pattern and the sidewall pattern as a mask can be considered. However, according to such a technique, it is necessary to expose a sidewall pattern portion by developing the wide pattern. For a positive resist, for example, the resist buried in the sidewall pattern portion needs to be removed by sensitizing for development. However, the sidewall pattern is formed in fine dimensions less than the resolution limit of exposure and thus, exposure light does not reach a resist near the lower portion of the sidewall pattern and the resist cannot be removed by development and thus, a resist residue arises. As a result, the resist residue is transferred also in the subsequent transfer of the sidewall pattern to a lower-layer film, posing a problem of a short of the sidewall pattern.
If a negative resist is used instead of a positive resist, there is no need to shine light on the sidewall pattern portion and the sidewall pattern portion can be exposed only by adjusting the developing time, but in this case, a problem of a collapsing sidewall pattern due to surface tension of water is caused in a rinsing/drying process after the development.
It is possible here to consider coexistence of a sidewall pattern and a wide pattern by forming a coating-type organic film on a wafer after the sidewall pattern being formed for planarization of steps by the sidewall pattern and then performing an optical lithography process of the wide pattern and etching the coating-type organic film using a resist as a mask. According to such a technique, however, a problem like the collapse of sidewall patterns by random air gaps generated by poor embedding between sidewall patterns when the coating-type organic film is coated arises. Also, a problem of difficulty of ensuring the needed thickness (height) of the sidewall pattern to process a lower-layer film when the coating-type organic film is etched arises.